Just my 2 cents, but I think one of the biggest causes of problems between a customer and the shop/mechanic is lack of communication or a car owner who has no mechanical knowledge being unable to comprehend what they are told. This is especially true at a car dealer where you have service writers who often do not have much, if any, more mechanical savvy than the customer.
Before wading into any repair try to get as much detail on the symptoms as possible. One common complaint is “Car won’t start”. Well, what does that mean? Starter motor turns the engine over but it won’t fire up, won’t crank at all, won’t click, etc, etc.
Try to reason a repair through before replacing anything. What I’ve often done is eat a bit of free time if that’s what it took to avoid guessing. That may be a bit distasteful working on flat rate with no guarantee but could prevent problems later on.
Always cover your bases with notes. A number of customers will have one repair done and blame everything currently wrong or that will go wrong in 2 years on the repair you just completed.
In theory, service managers and writers should head this kind of thing off before it ever gets to you but a lot of times that does not happen.